An example of conventional technology is explained with reference to FIGS. 5 and 6.
A multi-signal generator used for testing receivers of mobile communication systems outputs signals having multiple frequencies by adding a plurality of signals corresponding to the requirements of testing.
The configuration and its function of such a conventional technology are explained in the case where a multi-signal generator of three channels generates signals having three different frequencies.
As shown in FIG. 5, a multi-signal generator 1 in the conventional technology has a three-channel configuration and includes frequency synthesizers 11, 12, and 13, modulators 21, 22, and 23, amplifiers 71, 72, and 73, and attenuators 81, 82, and 83. The multi-signal generator 1 also includes an adder 90.
Each of the components and functions is explained in the following:
The frequency synthesizer 11 in the channel 1 is a variable frequency synthesizer in the frequency range of 0-2 GHz, and generates a frequency f1 as a carrier signal.
The modulator 21 is a mobile communication modulator used in a mobile communication system such as a quadrature modulator.
The quadrature modulator modulates an amplitude and phase of a carrier signal by an I channel and an Q channel of the base band signal.
Namely, the same phase (I: In Phase) components of the carrier signal is modulated by the binary data of an I channel base band signal, and the 90-degree phase shifted (Q : Quadrature Phase) components is modulated by the binary data of a Q channel base band signal, and then both modulated components are combined with one another to be output as a modulated carrier signal.
The modulated carrier signal is amplified by the amplifier 71 and is attenuated to a desired power level by the attenuator 81. Thus, the signal frequency f1 which is in the range of 0-2 GHz with a desired power level is output to the adder 90 as an output signal of the channel 1.
In the similar manner, an output signal of channel 2 and an output signal of channel 3 are provided to the adder 90.
These signals of three channels are combined by the adder 90 which outputs a signal having frequencies f1, f2, and f3 in the frequency range of 0-2 GHz.
In this example, however, when frequency variable synthesizers of wide frequency range and quadrature modulators are used, depending on frequency settings in the output signals, calibration processes become necessary to suppress spurious responses at the output of quadrature modulators by fine adjusting amplitudes and phases of the base band signals to modulate the carrier signals or several additional quadrature modulators must be provided.
In other words, in this method, the circuit configuration needs to be complex because the wide range frequencies are applied to the quadrature modulators.
Moreover, in the multi-signal generator using this method, the production cost needs to be high because the frequency synthesizers with wide variable frequency range and the modulators of wide frequency range are needed for the number of channels of the multi-signals.
To solve this problem, it may be possible to limit the variable frequency range of the frequency synthesizers 11, 12 and 13 to a narrower range of, for instance, 800 MHz.
However, it is difficult to deal with different test situations where frequency bands of the communication devices to be tested are changed, for example to 1.5 GHz or 1.7 GHz.
Another conventional example of multi-signal generator using frequency conversion method is explained with reference to FIG. 6.
This example has a 3-channel configuration having fixed frequency synthesizers 16, 17, 28, 36, 37 and 38, modulators 21, 22 and 23, mixers 61, 62, 63, 66, 67 and 68, variable frequency synthesizers 41, 42 and 43, band pass filters 51, 52 and 53, amplifiers 71, 72 and 73, attenuators 81, 82 and 83, and an adder 90.
The operation of this generator is explained for an example wherein a frequency range of 0-2 GHz for a signal frequency f1 is to be generated.
In the channel 1, the carrier frequency of the fixed frequency synthesizer is set to, for example 200 MHz, which is modulated by the modulator 21.
The modulated signal from the modulator 21 and an oscillation frequency 3.8 GHz of the fixed frequency synthesizer 36 are frequency converted by the mixer 61, thereby producing an IF output of frequency 4 GHz which is the sum of the two frequencies.
The frequency 4 GHz and a frequency 4-6 GHz of the variable frequency synthesizer 41 are frequency converted by the mixer 66, thereby producing an output signal of frequency 0-2 GHz which is the difference between the two frequencies.
The output signal is amplified by the amplifier 71, and the amplitude of which is adjusted to a desired level by the attenuator 81.
Similar to the foregoing example, the outputs of channel 1, channel 2 and channel 3 are added by the adder 90, thereby producing the multi-signals having three different frequencies f1, f2 and f3 with the desired level within the frequency range of 0-2 GHz.
In this configuration, two stages of mixers are configured so that the pass band property of the band pass filters need not be sharp.
More specifically, spurious responses are produced at the output of the mixers because of mutual modulation.
To suppress the spurious response, each local frequency for each of the two mixer is determined in such a way that the spurious response will not be produced in the vicinity of the frequency of the signal to be used.
In this configuration, the calibration is easily carried out because the frequencies of the frequency synthesizers 16, 17 and 18 and modulators 21, 22 and 23 are fixed as well as they are low enough.
However, the wide and variable frequency range such as 4-6 GHz is necessary for the frequency synthesizers 41, 42 and 43. Thus, the overall cost becomes high because such wide range and high frequency synthesizers must be configured by using YIG oscillators.
Here, a YIG oscillator is an oscillator which can oscillate in a wide frequency range by controlling an exciting current for an electric magnet which creates a direct current magnetic field applied to a single YIG crystal sphere.
Moreover, the overall cost of the generator further increases because the wide range frequency synthesizers 41, 42 and 43 are required for all the channels in order to output the multi-signals.
As explained in the foregoing, in the multi-signal generator in the first conventional example, the method which combines the variable frequency synthesizers and the quadrature modulators has a disadvantage which requires a wide range calibration capability.
When the fixed frequency synthesizers are alternatively used, it is difficult to deal with the changes in the frequency range or band of the communication devices to be tested.
The multi-signal generator of the second example involves a disadvantage in that the overall cost increases because the frequency synthesizers 41, 42 and 43 with high and variable frequency in the range of 4-6 GHz are required corresponding to the number of channels.
The present invention has been made to solve these problems involved in the conventional technology. The purpose of the invention is to provide a multi-signal generator which is capable of decreasing the cost by performing the modulation at low frequencies as well as commonly using an output of a frequency synthesizer.